In general, a direct current motor (hereinafter referred to as a DC motor) comprises a plurality of interpoles in order to improve the commutating characteristics of the DC motor. The interpoles are disposed at the intermediate position along the outer circumference of the armature between the adjacent main magnetic poles. The interpoles face the armature windings existing in commutating zones. The interpoles are interlinked by magnetic field produced by the armature reactions, and accordingly the interpoles are necessary to produce a magnetomotive force to eliminate the magnetic field produced by the armature reactions, in addition to a magnetomotive force to produce a magnetic field for commutation.
In order to reduce the magnetomotive force produced by the interpoles and to improve various operating characteristics of the DC motor, a DC motor having E-shaped interpoles has been proposed in U.S. patent application No. 884,586, now U.S. Pat. No. 4,220,882, by the applicant of the present invention. The E-shaped interpoles are disposed at the intermediate position between the adjacent main magnetic poles and attached to the inner circumference of a yoke by using nonmagnetic members. Each of the E-shaped interpoles comprises a center pole having an interpole winding wound thereon, and two legs which are disposed in front and rear of the center pole along the direction of the rotation of the armature, and the end surfaces of the center pole and the two legs face the armature with a small gap therebetween.
With reference to the accompanying FIG. 1, the conventional DC motor having the E-shaped interpoles will be explained in more detail. The conventional DC motor in FIG. 1 comprises an armature 1, a cylindrical shaped yoke 7 and two main magnetic poles 3 and 4 equidistantly spaced around the inner circumference of the yoke 7 and located outside of the armature 1. The main magnetic poles 3 and 4 have field windings 5 and 6 wound thereon, respectively. The field windings 5 and 6 are supplied with electric current in a predetermined direction so that the polarities of the main magnetic poles 3 and 4 are selected to be, for exmple, N and S, respectively. As a result, the armature 1 is counter clockwisely rotated as shown by arrow a. Some of the armature windings 21, 22 and 23, 24, which are located betweem the main magnetic poles 3 and 4, are within commutating zones.
In order to eliminate the counter electromotive force induced in the armature windings 21 through 24 within commutating zones, the E-shaped interpoles 8 and 9 are attached to the inner circumference of the yoke 7 via spacers 25 and 26 made of non-magnetic material, and are located at the intermediate positions along the outer circumference of the armature 1 between the main magnetic poles 3 and 4. The interpole 8 comprises a center pole 81 having an interpole winding 10 wound thereon, the two legs 82 and 83 disposed respectively in front and rear of the center pole 81 along the direction of the rotation of the armature 1. The interpole winding 10 is connected in series with the armature winding 2, and an armature current passes through the interpole winding 10 in such a direction so that the polarity of the center pole 81 becomes S and the polarity of the legs 82 and 83 becomes N. The other interpole 9 also comprises a center pole 91 having an interpole winding 11 wound thereon, and two legs 92 and 93. The interpole winding 11 is connected in series with the armature winding 2, and the direction of the armature current passing through the interpole winding 11 is selected so that the polarity of the center pole 91 becomes N and the polarity of the legs 92 and 93 become S.
The E-shaped interpoles 8 and 9 are hardly affected by the magnetic flux produced by the armature reactions. This is because the magnetic flux caused by the whole of the armature current flowing through the armature winding 2 hardly penetrates the interpoles 8 and 9 due to the existence of the spacers 25 and 26 of non-magnetic material. For example, in the interpole 8, only the magnetic flux f.sub.1 and f.sub.2, which is caused by the current passing through the armature windings 21 and 22 within a commutating zone, pass through the magnetic circuit including the center pole 81, legs 82 and 83 of the interpole 8 and the armature 1. Thus, the amount of the magnetomotive force produced by the E-shaped interpoles 8 and 9 can be greatly reduced, and therefore, the cross sectional area of the interpole winding can be very small and the heat generated by the interpoles can be reduced.
However, since each of the E-shaped interpoles has two legs in addition to the center pole as mentioned before, the length of the E-shaped interpole, which faces the armature along the outer circumference of the armature is relatively large, and thus the length of the pole pieces 34 and 44 of the main magnetic poles 3 and 4 along the outer circumference of the armature 1 cannot be sufficiently large. In the DC motor illustrated in FIG. 1, an axis A, which passes through the centers of interpoles 8, 9 and the center of rotation 0 of the armature 1, constitutes an interpole axis or a geometrical neutral axis, and an axis B, which is at a 90.degree. electrical angle with the axis A and passes through the center of rotation 0 of the armature 1, constitutes a direct axis. The electrical angle of 90.degree. corresponds to a 90.degree. geometrical angle in a two pole motor, and a 45.degree. geometrical angle in a four pole motor, and so on. The direct axis B in the DC motor of FIG. 1 coincides with a line which passes through the centers of the main magnetic poles and the center of rotation 0 of the armature 1. End portions 31 and 32 of the pole piece 34 of the main magnetic pole 3 are located in a symmetrical position with regard to the direct axis B. Therefore, the angle between the direct axis B and a line which passes through the center of the rotation 0 of the armature 1 and the end portion 31 or 32 is defined as .alpha., and thus the length of the pole piece 34 along the outer circumference of the armatuare 1 is directly proportional to an angle 2.alpha..
In the conventional DC motor illustrated in FIG. 1 having the E-shaped interpoles, the angle 2.alpha. cannot be sufficiently large because of the presence of the E-shaped interpoles as aforementioned. Therefore, each area of the surfaces of the pole pieces 34, 44 of the main magnetic poles which face the armature 1 cannot be sufficiently large, and the amount of the magnetic flux of the main magnetic poles which passes through the armature 1, and interlinks with the armature winding 2 is limited to a relatively small value so that the torque of the DC motor cannot be sufficiently large.